Heretofore, various compositions have been proposed to provide protection for corrodible metallic surfaces. In recent years, considerable attention has been directed to compositions and systems for providing cathodic corrosion protection for metallic structures. Such systems and compositions are applicable for protecting underground pipes, storage tanks and the like, as well as metallic surfaces in contact with water, such as ship hulls, support structures for drilling rigs, docks, and the like.
In general, such systems or compositions utilize either an external source of electrical current which maintains the surface to be protected cathodic, or the corrosion protection composition itself forms an internal battery with the metal surface to be protected. In these latter systems, the coatings contain metal particles which are more anodic than the metal surface to be protected, and thus act as sacrificial anodes. In the most common coatings of this latter type, the composition is constituted by a binder and a filler. The binder may be any suitable organic or inorganic binder material and the filler is constituted by conductive metal particles which are more anodic than the metal which is to be protected. Most frequently, the metal particles of such coatings are zinc particles.
While such coating compositions have recently been used to a considerable extent, it has been found that the metal particles, such as zinc dust or powder, add greatly to the cost of such composition. Moreover, it is frequently necessary to utilize appreciable amounts of this relatively expensive metallic filler, e.g. 80% or more by weight of the total composition in order to provide the requisite corrosion protection of the metal surface.
The problem of the high cost of the metallic filler was solved by the replacement of a substantial portion thereof with a comminuted refractory ferroalloy for example ferrophosphorus. As disclosed in U.S. Pat. No. 3,562,124, it was found that at least about 3% and preferably from about 10 to about 85% of the zinc, or other conductive metal particles can be replaced by the comminuted particles of a refractory ferroalloy and that the resulting composition is not only less expensive than the conventional zinc rich coating composition but also in most instances, possesses enhanced corrosion protection effectiveness. It has been found however that the metal substrates coated with the prior art corrosion resistant coatings can be welded effectively only with some difficulty. That is to say, the areas proximate to the weld are no longer protected from corrosion, the continuity of the weld is poor and the electrode life is relatively short.
Inasmuch as these coating compositions are designed for use in connection with underground pipes, storage tanks and the like wherein the pipes and appurtenances are welded in place, it can be seen that the coated pieces should be weldable as well as protected from corrosion.